Integrated circuits are made using silicon wafers. The silicon wafers are processed into integrated circuits using a variety of techniques to form transistors and other circuit elements upon the silicon wafer. These processes involve multiple steps in which layers of materials are selectively deposited and/or selectively etched away. One of the steps commonly used is Chemical Vapor Deposition (CVD), in which a very thin layer of a metal or other material is deposited upon the surface of the silicon wafer. Low Pressure Chemical Vapor Deposition (LPCVD) is a process in which chemical vapor deposition takes place under a vacuum or a partial vacuum.
CVD machines generally include multiple stations at which sequential depositions or other actions are performed. Because of the small size of the traces needed for integrated circuits, proper and consistent alignment and registration, and the uniform deposition of materials, are crucial to the quality of the finished product.
CVD devices must carefully control the distribution and flow characteristics of the fluid which includes the metal or other material deposited. In silicon wafer technology, the fluid is generally admitted into the chamber of a fluid distribution head through an opening which terminates an inlet tube. The fluid stream encounters a deflector plate within the chamber which acts to disperse the fluid stream throughout the distribution head. The dispersed fluid then exits the distribution head through a multiplicity of openings in a perforated plate. The axis of the perforated plate is generally perpendicular to the axis of the inlet tube.
Temperature within the distribution head and at the surface of the wafer must be carefully controlled. For example, when a metal oxide film is deposited onto a silicon wafer using a Novellus Concept One (Novellus Systems, Inc, San Jose, Calif.) LPCVD machine, the temperature at the wafer is maintained at 400.degree. C. .+-.8.degree. C. Extreme temperatures such as these can cause the perforated plate to deform, and to deflect from the planar configuration. When the perforated plate becomes deformed it alters the distribution pattern of the fluid distribution head, causing the fluid to be unevenly applied to the silicon wafer. When deformation becomes excessive it can also disrupt the wafer registration, so that the silicon wafer misfeeds across the fluid distribution head. Either of these occurrences can cause the completed integrated circuit to fail.
Maintaining fluid distribution heads in an undeformed configuration has proven to be a problem which has been difficult and expensive to solve. For example, the eight fluid distribution heads which were original equipment in a commercially available LPCVD device demonstrated deformation after only three months of use. All eight heads must be replaced as a set, at a cost of $10,000 to $12,000 per set. The LPCVD machine must be shut down while the replacement takes place, with attendant repair and down-time costs.
One approach to prolonging the useful life of the fluid distribution heads has been to thicken the perforated plate. While this has prolonged the useful life of the heads somewhat (bowing takes place after four to six months rather than after three months) it has not solved the deformation problem, or reduced the expense of frequent replacement of the fluid distribution heads.